Inorganic luminescent or electromagnetically active materials are crystalline compounds that absorb energy acting upon them and subsequently emit the absorbed energy. Light emission is known as luminescence. A material which continues to emit light for greater than 10.sup.-8 seconds after the removal of the exciting energy is said to be phosphorescent. Phosphorescent substances are also known as phosphors, and as lumiphors. In contrast to phosphorescent substances, substances in which the emission of light ceases immediately or within 10.sup.-8 seconds after excitation are said to be fluorescent substances. The half-life of the afterglow of a phosphor will vary with the substance and typically ranges from 10.sup.-6 seconds to days.
Phosphors may generally be categorized as stokes (down-converting) phosphors or anti-stokes (up-converting) phosphors. Phosphors which absorb energy in the form of a photon and transmit a lower frequency band photon are down-converting phosphors. In contrast, phosphors which absorb energy in the form of two or more photons in a low frequency and emit in a higher frequency band are up-converting phosphors. Phosphors may also be categorized according to the nature of the energy which excites the phosphor. For example, phosphors which are excited by low energy photons are called photoluminescent and phosphors which are excited by cathode rays are called cathodluminescent. Other electromagnetically active particles include pigments and radio frequency absorbers.
Phosphors are employed in a wide variety of applications. Such applications include, but are not limited to, coding of mass-produced goods or high value trademarked articles, printing inks, biological assays, general lighting, safety lighting, x-ray machines, cosmetic dentistry and in cathode-ray tubes, such as in television and computer monitor screens. These and other applications are described in more detail in Luminescent Materials, Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A15, pgs. 519-557, the disclosure of which is incorporated by reference herein.
The requirements for phosphors have become more and more stringent with smaller and smaller phosphor particles being required. For example, phosphor particles may be used in ink compositions for use in ink jet printers which requires the coating or encapsulation of the phosphor particles such that they can be suspended in an ink formulation. Phosphor particles are required to have a diameter in the range of one micron or less in order to be suspended in an ink formulation. Computer monitors are another example of the need for smaller phosphor particles. Computer monitors require higher resolution than conventional television screens. In order to achieve higher resolutions, smaller phosphor particles are often needed. Biological assays are another example of an application which requires monodispersity as well as small particle size phosphors. Current methods of producing phosphor particles need significant improvement to meet the challenges presented in the production of small phosphor particles.
Typically, phosphor particles are produced by first precipitating amorphous, generally spherical, unactivated, phosphor particles from solution. These precipitated particles are unactivated phosphors in the sense that they exhibit little or no phosphorescence. To improve the phosphorescence of the precipitated particles, the precipitated particles have been fired in fixed bed furnaces, optionally in the presence of reactive fluxes.
The unactivated phosphor particles are fired in a fixed bed furnace at temperatures ranging from 900 to 1600.degree. C. in order to change the crystalline lattice structure of the particle. The fixed bed furnace normally comprises a means of heating a crucible or boat in a closed environment, such as an alumina-lined quartz tube, a cold wall quartz reactor or a quartz-lined reactor. The types of materials used for the crucibles and boats have an important effect upon the formation of the activated phosphor. For example, boron nitride, molybdenum and nylon capped boats have been attempted but cause metal impurity contamination. In contrast, quartz and alumina boats and platinum crucibles have generally proven satisfactory to prevent such contamination. Unfortunately, firing the unactivated phosphor particles in a fixed bed causes the particles to agglomerate, increasing the overall particle size.
Optionally, a reactive flux may be present in the fixed bed furnace. A reactive flux material is a substance that promotes the fusing of the phosphors and the reactive material contained in the flux. A reactive flux can also promote activation of the unactivated phosphor particles at lower temperatures. The reactive flux material may be a gas, liquid or solid. However, the presence of a reactive flux in a fixed bed process requires an additional process step to remove the flux from the final phosphor particle. Removing the reactive flux often deteriorates the surface of the phosphor particles and impairs the phosphor's efficiency. Furthermore, the reactive fluxes often causes the phosphor particles to fuse together yielding agglomerated phosphor particles rather than monodisperse phosphor particles.
As mentioned above, phosphor particles which have been fired in a fixed bed, especially those fired in the presence of a reactive flux, tend to form hard agglomerates. Typically, fixed bed phosphor production methods produce less than about 0.1% monodisperse submicron phosphor particles. The agglomeration substantially increases the overall particle size. In order to achieve the desired smaller particle sizes, phosphor particles formed by the fixed bed process are milled or crushed. These crushed particles are then sieved such that a particle size of phosphor may be obtained. However, the milling or crushing of phosphor particles into smaller sizes often results in a decrease in their phosphorescent properties.
When phosphor particles are milled or crushed the crystalline lattice structure of the phosphor particle can become fractured and exhibit lower emission conversion efficiency. For example, crushing a phosphor particle to reduce size can decrease the efficiency of the phosphor by 75% as compared to the uncrushed phosphor particle. Indeed, the yield of useable submicron phosphor material can be less than 1% after firing unactivated phosphor particles in a fixed bed followed by milling the resulting activated phosphors. Thus, there exists a need in the art for a process which is capable of producing activated phosphor particles which do not form agglomerates requiring milling or crushing.